How to Stop a Crack in Wood From Spreading

No matter how careful you are, cracks in wood happen. I reviewed four methods for dealing with cracked wood. 

Stopping Cracks in Wood From Spreading

During the drying process, wood shrinkage leads to cracking, checking, and splitting. Wood that has been properly kiln-dried is less likely to split or crack than green wood. Since preventing cracks is impossible, it is important for any woodworker to know how to deal with cracks once they appear. 

Remove Cracks on the Edges of Unused Stock

Before you start working with the wood you’ve chosen for your next project, inspect the edges and ends for cracks. Since cracks tend to spread, the best choice is simply to remove the cracked portion of the wood.

1. Mark your cut. Use a straightedge and a pencil to draw a cut line that is even with the end of the crack. Removing cracked material will prevent the wood from warping. 
2. Cut away the crack. Use a miter saw or a table saw with a crosscut sled to carefully cut along your pre-marked line. To prevent the crack from spreading through the rest of the wood, you must be certain to remove all the cracked material. 
3. Inspect the end. The crack may extend several inches beyond what you could see on the surface. If the crack is still visible in the interior of the plank or board, try inserting something thin and narrow to see how deep it goes. You can then mark a new cut line and cut again. 

Patch Small Cracks With Sawdust and Wood Glue

For a perfect color match, you can use a sawdust and wood glue mixture to fill cracks and keep them from spreading. 

1. Collect or create sawdust. If you have a sander with a collection bag, sand some of the stock to produce clean and fresh sawdust. You can also sand by hand. Lay down newspaper to catch the sawdust and make sure it doesn’t mix with any dirt or dust. 
2. Add wood glue. Scoop sawdust into a clean container and add the glue. White wood glue is best for color matching. Stir the mixture with a craft stick. If the mixture is runny, add more sawdust. The glue and sawdust should form a dough-like paste that can be rolled by hand.
3. Fill the crack. Roll the doughy putty into a cylinder. Push the paste into the crack. Wood glue hardens in ten minutes or less, so you need to work quickly. Use a putty knife to scrape away the excess and smooth the surface of the wood glue/sawdust mixture.
4. Wait for the glue to dry. While the glue will start to harden in about ten minutes, it takes between six and 24 hours to fully cure. The packaging will indicate how much time is needed for curing. 
5. Finish. Lightly sand the filled crack with fine-grit sandpaper. Avoid applying too much pressure. The filler is softer than the wood around it, and can be easily gouged. 

Fill Large Cracks with Epoxy Resin

When a large crack mars the surface of your workpiece, epoxy resin can fill the gap while preventing the crack from getting larger. Not only does it work as a filler, epoxy resin is also an adhesive, so it will hold the sides of the crack together and prevent further damage.

1. Prepare your workspace. Epoxy resin takes 24 hours to fully cure. Before that time, it will act like water, flowing to the lowest possible point. Use masking tape on the bottom and ends of cracks to keep your epoxy resin from running off the board and onto your floor. Lay down newspapers to protect your workspace and the floor below, just in case. 
2. Plan your ‘open time’. After mixing, epoxy resin has a brief window of liquidity referred to as the ‘open time’. Epoxy resin congeals during the initial cure phase, before fully hardening. The open time varies from 5-60 minutes and should be listed on the package. 
3. Mix the epoxy resin. Epoxy resins come in two parts. One part resin is mixed with one part hardener. Use a clean container, and only mix as much epoxy resin as you will use in one session. Once mixed, epoxy resin can not be stored. 
4. Tint the epoxy resin. Powdered pigments can be mixed into the liquid epoxy resin to color the final product. To hide the crack in your wood, choose tints that are close to the color of the wood. You can also choose other colors to highlight or contrast the undertones present in the wood, turning the crack into a work of art. 
5. Fill the crack. Pour the epoxy resin into the crack and allow it to fill the empty space. Fill all the way to the top of the crack. It is better to overfill slightly than to underfill. Any excess can be sanded away after the epoxy resin cures. 
6. Cure the epoxy resin. Most epoxy resins take about 24 hours to fully cure. Epoxy resin cures best at temperatures slightly higher than 75 degrees Fahrenheit. The temperature should be stable throughout the entire curing process, so it is best not to leave your workpiece in a garage or non climate-controlled space. 
7. Sand away excess. Use a sanding block and 60 to 80-grit sandpaper to level out the epoxy resin. Follow with 180 to 220-grit sandpaper to smooth the surface. For a super smooth finish, lightly sand with 320-grit sandpaper. 
8. Finish. Apply a light coat of oil or a polishing wax to the resin and the wood, using a clean, lint-free cloth. You can also apply varnish for additional protection. 

You can also use wood filler for large holes, but it isn’t as strong as epoxy.

Reinforce the Crack with Bowtie Inlays

This technique is fairly advanced, but templates make the job easier. Bowtie-shaped inlays are an efficient and impressive way to reinforce cracks in the middle of wood. 

1. Prepare your materials. You will need a router with a plunge base and an inlay kit. The kit should come with several templates, and a guide bushing that attaches to the base of your router. Attach the guide bushing. 
2. Rout the mortise. Use double-sided tape to stick one of the templates to the wood. The narrowest part of the bowtie should be centered over the crack. Set the desired depth and follow the lines of the template. Continue removing material until you have a flat-bottomed, bowtie-shaped depression in the wood, crossing the crack. 
3. Rout the bowtie. Remove the guide bushing. Stick the same template to the wood you are using for your inlay. The wood should be the same thickness, or slightly thicker, than the depth of your mortise. Rout along the outer edge of the template. The bowtie inlay should pop out.  
4. Insert the inlay. Apply wood glue to all contact surfaces. Press the inlay into the mortise and wipe up any excess glue. Sand off any excess after the glue has completely dried. 

Can I Use Wood Putty to Stop Cracks from Spreading? 

Wood putty is not an adhesive, and will not prevent the crack from getting larger. Wood putty is great for filling in small gaps where epoxy resin won’t flow. It can be precisely color-matched, making it one of the most effective ways to hide a small crack.

Conclusion

Keep cracks in wood from spreading using one of four solutions provided above. Remove cracked wood before starting your project. Patch small cracks with wood glue and sawdust. Fill large cracks with epoxy resin, or reinforce split wood with bowtie inlays.